Fulltone OCD debug

[the3secondrule]

Hey guys,

i've had a v4 OCD come across my desk, with no output

Voltages:
1  8.63 (this seems high - should be VB?)
2  5.1
3  5.5
4  0
5  7.1
6  6.3
7  6.3
8  9.3

audio-probing I get signal up to pin 3, and it's all quiet after that.

this was a working fulltone, (not a clone), that I sold to a friend that then returned to me a few months later, dead.
any ideas?

[mgwhit]

Yeah, pins 1-3 and 5-7 should be at or around VB, which should be a little less than half of the 9.3V supply voltage.  First thing I'd do is test the resistors in the voltage divider to see if anything is off with them, including checking to see if the one that should connect to ground has good continuity to ground.  Not that I think anything would happen in that part of the circuit, but it's easy to test and who knows, right?  After that, I'd swap out the op amp and see if that helps.  I'd bet it's gone kerflooie.  Put in a socket while you're in there.  Good luck and let us know what you find!

[the3secondrule]

Yeah, I suspected the chip as well, so I popped in a socket and replaced it with a 4558 (not ideal - but it was the closest thing to hand), and no joy - still silent.

Will check the resistors in the voltage divider...

[mgwhit]

Hmmm.  Have you checked the voltages on the socket pins to see what they are with no IC inserted?

[the3secondrule]

I'll check tonight and report back..

[the3secondrule]

voltages sans chip:

1  3.2v
2  1.3v
3  4.1
4  0
5  3.8
6  0.5v
7  0.5v
8  9.4

[mgwhit]

Here are the voltages from my EgoDriver (v5).  The Main difference should just be a slightly lower supply voltage thanks to an additional 100R resistor on the way to pin 8.

With TL082:

4.27 - 1    8 - 8.52
4.27 - 2    7 - 4.27
4.07 - 3    6 - 4.27
   0 - 4    5 - 4.26

Without TL082:

4.04 - 1    8 - 8.93
4.02 - 2*   7 - 0.42
4.27 - 3    6 - 0.42
   0 - 4    5 - 4.04

* Pin 2 varies inversely with the Gain setting without the IC in place.  I got 4.04V at the lowest Gain setting and 3.85V at the highest.  I forgot to experiment with the Gain knob with the IC in place.  I'm also pretty sure I used an A500K Gain pot rather than the stock A1M.

So your op amp reference voltages are way over spec when the IC is in place, and somewhat under spec (especially op amp 1 on pins 1-3) with the IC out.  Thinking cap time.

How do the 10uF electrolytics look?